During the early stages on embryonic development, the formation of the embryonic axis is initiated at one pole of the embryo. Most of what is known about the molecular mechanisms underlying this process is based on amphibian embryos, where maternally-derived determinants control this polarity. In amniotes (birds and mammals) this is unlikely because embryos at much later stages can initiate axis formation at any position. Here, the mechanisms that determine embryonic polarity in the early stages of development will be studied using the chick embryo as a model, because of their ease of manipulation and culture. First, the role of the hypoblast (which had been suggested to be responsible for polarity determination) will be tested directly. Then, the region homologous to the amphibian "Nieuwkoop center" (which has been shown in frogs to determine the position of future "organizer" and axial cells) will be located. Finally, a number of genes already found to be expressed in a way that predicts the future polarity of the embryo will be placed in hierarchial order, taking advantage of the fact that portions of the embryo will initiate axis formation when isolated from the rest of the embryo. The results of this project will be of value in suggesting how the molecular cascades that have been implicated in the early development of lower vertebrates (fish, amphibians) can be applied to higher vertebrates (birds and mammals). Potentially, knowledge of these cascades could lead to the development of new molecular diagnostic tools for some of the many serious congenital disorders that affect the early development of the embryonic axis, and may help determine whether monozygotic and conjoined twinning are likely to have a molecular basis.